Radar plow drillstring steering

ABSTRACT

A radar-plow drillstring steering system comprises a steering plow and a measurements-while-drilling instrument for mounting just behind the drill bit and downhole motor of a drill rod. The instrument includes a radar system connected to upward-looking and downward-looking horn antennas and a dielectric-constant sensor. The steering plow includes four pressure pads radially distributed around the outside surface and their associated servo motors. A coordinated control of the pressure pads allows the steering plow to push the drillstring and drill bit up-down-left-right. The antennas and sensor are embedded in respective ones of the pressure pads and are used to electronically and non-invasively probe a coal seam to locate its upper and lower boundary layers. The dielectric-constant sensor provides corrective data for the up and down distance measurements. Such measurements and data are radio communicated to the surface for tomographic processing and user display. The radio communication uses the drillstring as a transmission line and F 1/ F 2  repeaters can be placed along very long runs to maintain good instrument-to-surface communication. A docking mechanism associated with the instrument and its antenna array allows the instrument to be retrieved back inside the drillstring with a tether should the drill head become hopelessly jammed or locked into the earth.

RELATED APPLICATION

[0001] This Application is a continuation-in-part of both U.S. patentapplication Ser. No. 09/820,498, filed Mar. 28, 2001, and titledGROUND-PENETRATING IMAGING AND DETECTING RADAR; and, U.S. patentapplication Ser. No. 10/161,378, filed Jun. 4, 2002, and titledSHUTTLE-IN RECEIVER FOR RADIO-IMAGING UNDERGROUND GEOLOGIC STRUCTURES.The latter of which further claims priority by virtue of U.S.Provisional Patent Applications, serial No. 60/315,149, filed Aug. 27,2001, and titled RADAR-NAVIGATION TOOL FOR MINING COAL, and serial No.60/335,520, filed Oct. 31, 2001. Such Applications are incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates generally to directional drilling ofboreholes in coal seams, and more specifically to steering mechanisms,radars and control systems for drilling horizontal boreholes in coaldeposits.

[0004] 2. Description of the Prior Art

[0005] The production of coal and methane depends upon the environmentof the original coal bed deposit, and any subsequent alterations. Duringburial of the peat-coal swamp, sedimentation formed the sealingmudstone/shale layer overlying the coal bed. In deltaic deposits,high-energy paleochannels meandered from the main river channel.Oftentimes, the channels scoured through the sealing layer and into thecoal seam.

[0006] High porosity sandstone channels often fill with water. Under thepaleochannel scour cut bank, water flows into the face and butt cleatsof the coal bed. Subsequent alterations of the seam by differentialcompaction cause the dip, called a roll, to occur in the coal bed.Faults are pathways for water flow into the coal bed.

[0007] Drilling into the coal bed underlying a paleochannel andsubsequent fracking can enable significant flows of water to enter. Thecurrent state of the art in horizontal drilling uses gamma sensors in ameasurements-while-drilling (MWD) navigation subsystem to determine whenthe drill approaches a sedimentary boundary rock. But if sandstone isprotruding into the coal, such as results from ancient river bed cuttingand filling, then the gamma sensor will not help. Sandstone does nothave significant gamma emissions, so this type of detection isunreliable. Drilling within the seam cannot be maintained when the seamis not bounded by sealing rock.

[0008] Methane diffusion into a de-gas hole improves whenever thedrillhole keeps to the vertical center of the coal seam. It alsoimproves when the drillhole is near a dry paleochannel. Currenthorizontal drilling technology can be improved by geologic sensing andcontrolling of the drilling horizon in a coal seam.

[0009] One present inventor, Larry G. Stolarczyk, has described methodsand equipment for imaging coal formations in geologic structures in manyUnited States patents. Some of those patents are listed in Table I, andare incorporated herein by reference. TABLE I Patent No. Issued Title4577153 Mar. 18, 1986 Continuous Wave Medium Frequency SignalTransmission Survey Procedure For Imaging Structure In Coal Seams4691166 Sep. 01, 1987 Electromagnetic Instruments For Imaging StructureIn Geologic Formations 4742305 May 03, 1988 Method For ConstructingVertical Images Of Anomalies In Geological Formations 4753484 Jun. 28,1988 Method For Remote Control Of A Coal Shearer 4777652 Oct. 11, 1988Radio Communication Systems For Underground Mines 4879755 Nov. 07, 1989Medium Frequency Mine Communication System 4968978 Nov. 06, 1990 LongRange Multiple Point Wireless Control And Monitoring System 4994747 Feb19, 1991 Method And Apparatus For Detecting Underground ElectricallyConductive Objects 5066917 Nov. 19, 1991 Long Feature Vertical OrHorizontal Electrical Conductor Detection Methodology Using PhaseCoherent Electromagnetic Instrumentation 5072172 Dec. 10, 1991 MethodAnd Apparatus For Measuring The Thickness Of A Layer Of GeologicMaterial Using A Microstrip Antenna 5087099 Feb. 11, 1992 Long RangeMultiple Point Wireless Control And Monitoring System 5093929 Mar. 03,1992 Medium Frequency Mine Communication System 5121971 Jun. 16, 1992Method Of Measuring Uncut Coal Rib Thickness In A Mine 5146611 Sep. 08,1992 Mine Communication Cable And Method For Use 5181934 Jan. 26, 1993Method For Automatically Adjusting The Cutting Drum Position Of AResource Cutting Machine 5188426 Feb. 23, 1993 Method For ControllingThe Thickness Of A Layer Of Material In A Seam 5260660 Nov. 09, 1993Method For Calibrating A Downhole Receiver Used In ElectromagneticInstrumentation For Detecting An Underground Conductor 5268683 Dec. 07,1993 Method Of Transmitting Data From A Drillhead 5301082 Apr. 05, 1994Current Limiter Circuit 5408182 Apr. 18, 1995 Facility And Method ForThe Detection And Monitoring Of Plumes Below A Waste Containment SiteWith Radiowave Tomography Scattering Methods 5474261 Dec. 12, 1995 IceDetection Apparatus For Transportation Safety 5686841 Nov. 11, 1997Apparatus And Method For The Detection And Measurement Of Liquid WaterAnd Ice Layers On The Surfaces Of Solid Materials 5769503 Jun. 23, 1998Method And Apparatus For A Rotating Cutting Drum Or Arm Mounted WithPaired Opposite Circular Polarity Antennas And Resonant Microstrip PatchTransceiver For Measuring Coal, Trona And Potash Layers Forward, SideAnd Around A Continuous Mining Machine RE032563 Dec. 15, 1987 ContinuousWave Medium Frequency Signal Transmission Survey Procedure For ImagingStructure In Coal Seams RE033458 Nov. 27, 1990 Method For ConstructingVertical Images Of Anomalies In Geological Formations

[0010] There are a number of conventional ways directional drills use tosteer in a desired direction. One involves placing the drill bit and itsdownhole motor at a slight offset angle from the main drillstring. Thewhole drillstring is then rotated to point the offset angle of the drillbit in the direction the operator wants the borehole to head. Anothermethod involves an articulated joint or gimbal behind the drill bit andits downhole motor and using servo motors to angle the joint for thedesired direction.

SUMMARY OF THE PRESENT INVENTION

[0011] Briefly, a radar-plow drillstring steering embodiment of thepresent invention comprises a steering plow and ameasurements-while-drilling instrument for mounting just behind thedrill bit and downhole motor of a drill rod. The instrument includes aradar connected to upward-looking and downward-looking horn antennas anda dielectric-constant sensor. The steering plow includes four pressurepads radially distributed around the outside surface and theirassociated servo motors. A coordinated control of the pressure padsallows the steering plow to push the drillstring and drill bitup-down-left-right. The antennas and sensor are embedded in respectiveones of the pressure pads and are used to electronically andnon-invasively probe a coal seam to locate its upper and lower boundarylayers. The dielectric-constant sensor provides corrective data for theup and down distance measurements. Such measurements and data are radiocommunicated to the surface for tomographic processing and user display.The radio communication uses the drillstring as a transmission line andF1/F2 repeaters can be placed along very long runs to maintain goodinstrument-to-surface communication. A docking mechanism associated withthe instrument and its antenna array allows the instrument to beretrieved back inside the drillstring with a tether should the drillhead become hopelessly jammed or locked into the earth.

[0012] An advantage of the present invention is that a drillstringsteering plow is provided for directional drilling.

[0013] Another advantage of the present invention is that a drillstringsteering plow is provided that keeps radar sensing antennas in intimatecontact with the media.

[0014] A further advantage of the present invention is a drillstringsteering system is provided that can be self-guided and is relativelyinsensitive to groundwater.

[0015] These and other objects and advantages of the present inventionwill no doubt become obvious to those of ordinary skill in the art afterhaving read the following detailed description of the preferredembodiment which is illustrated in the various drawing figures.

IN THE DRAWINGS

[0016]FIG. 1 is a diagram of a coal mine operation in an embodiment ofthe present invention that uses directional drilling andmeasurements-while-drilling radar to guide the drilling of horizontalboreholes within the vertical centers of coal deposits;

[0017]FIG. 2 is a cross sectional diagram of a coal deposit, similar toFIG. 1, in which a sandstone paleochannel is protruding from the ceilingof the coal deposit, and in which the drilling of a horizontal boreholehas dived down below the paleochannel to stay within the middle of thecoal;

[0018]FIG. 3 is a diagram of a radar-plow drillstring steering systemembodiment of the present invention;

[0019]FIG. 4 is a functional block diagram of a drillstring radarembodiment of the present invention, and shows the electronic componentsand their relationships as used in the equipment of FIGS. 1-3;

[0020] FIGS. 5A-5D are cross-sectional diagrams of the plow steeringpart of the equipment illustrated in FIG. 3, and shows a variety ofpositions of the pressure pads for drilling straight ahead (FIG. 5A),drilling-up (FIG. 5B), drilling-down (FIG. 5C, and drilling-left (FIG.5D);

[0021]FIG. 6 is a schematic diagram of a ground-penetrating radar systemembodiment of the present invention, and is one way to implement theradar electronics portion of the equipment illustrated in FIGS. 3 and 4;

[0022]FIG. 7 is a graph of the typical driving-point real and imaginaryimpedance variations seen in the system of FIG. 1 for variousthicknesses of a coal layer deposit; and

[0023]FIG. 8 is a graph of the theoretical impedance response of adirectional-coupler reflection port of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024]FIG. 1 illustrates a coal mine operation 100 in its earlieststages of development. An overburden 102 of soils covers a coal layer104. This in turn overlies a deeper substrate layer 106. The coal layer104 may be interrupted by a fault or scouring that happened overgeologic time and was filled with sandstone, e.g., a paleochannel 108.At a minimum, the coal layer will undulate and vary in thickness.

[0025] The sandstone in paleochannel 108 is porous and can be saturatedwith water. If so, flooding of the coal mine can occur from overhead ifthe coal layer just under the paleochannel is faulted or cut. It istherefore in the economic and safety interests of mining operations toavoid punching through such paleochannel 108 when taking the coal incoal layer 104.

[0026] The coal layer 104 will typically contain valuable reserves ofmethane gas that can be pumped out through a network of horizontallyrunning boreholes 110 and 112. These boreholes are begun as verticalbores 114 and 116 using a directional drilling method.

[0027]FIG. 2 illustrates a horizontal drilling operation 200. Anoverburden 202 sits on top of a coal bed deposit represented by an uppercoal deposit 204 and a lower coal deposit 206. These overlie a deepersubstrate 208. The coal bed deposit is preferably drilled such that ahorizontal borehole 210 is vertically equidistant between upper andlower coal deposits 204 and 206. In some situations, the horizontalborehole 210 may need to be lined with a perforated plastic pipe thatlacks perforations in the water saturated areas.

[0028] Such horizontal borehole 210 is drilled by a drillstring 212 thathas a steerable, directional drillhead 214. A paleochannel 216 filledwith sandstone and probably saturated with water represents a commonhazard in such mining. FIG. 2 shows that the horizontal borehole 210 hasbeen drilled to stay in the middle of whatever vertical space there isto work within the upper and lower coal deposits 204 and 206. Suchguidance while drilling requires a non-invasive probing of the depositsahead of the drilling that is provided by embodiments of the presentinvention.

[0029] A natural waveguide occurs in layered coal seam sedimentarygeology because there is a contrast between the electrical conductivityof the coal and that of shale, mudstone, and/or fire clay. Theconductivity of coal is about 0.0005 S/m (2,000 ohm-meters). So dry coalis a pretty good insulator. The typical bounding layers have electricalconductivity in the range of 0.01 to 0.1 Siemens per meter (S/m). Such20:1 conductivity contrast creates a natural waveguide, and any inducedelectromagnetic (EM) waves will travel along inside the coal bed layer.

[0030] An electric field (EZ) component of the traveling EM wave isvertically polarized, and the magnetic field (Hy) component ishorizontally polarized in the seam. The energy in this part of the EMwave travels laterally in the coal seam from a transmitter to areceiver, e.g., a Stolar Horizon Radio Imaging Method (RIM) receiver. Ahorizontally polarized electric field (EX) component has a zero valuenear the vertical center of the seam, and is maximum at the sedimentaryrock-coal interface. It is this EX component that is responsible for EMwave signal transmission into the boundary rock layer. The energy inthis part of the EM wave travels vertically in the coal deposit.

[0031] In such waveguide, the coal seam radiowave magnitude diminishesover distance for two reasons. The first is the attenuation rate of thewaveguide. The second is the radial spreading of wave energy as ittravels away from the transmitter. The cylindrical spread can bemathematically represented by $\frac{1}{\sqrt{r}},$

[0032] where r is the distance from the transmitting to receivingantenna. This compares with a non-waveguide far field sphericallyspreading factor of $\frac{1}{r} \cdot$

[0033] Thus, at hundred meters, the magnitude of the seam EM wavedecreases by a factor of only ten in the waveguide, and by a factor ofhundred in an unbounded media.

[0034] A significant advantage of the seam waveguide is signals cantravel farther. Another advantage is that the traveling electromagneticwave predominantly remains within the coal seam waveguide.

[0035] Such coal seam electromagnetic wave is very sensitive to changesin the waveguide geology. The radiowave attenuation rate and the phaseshift can be measured and the measurements will reveal structuralfeatures of the coal seam. If the waveguide were uniform, then the pathwould be a straight line. The straight line path is an assumption usedin the Algebraic Reconstruction Technique (ART) tomography algorithm.But radiowaves are refracted near significant geologic anomalies causingthe travel path of the radiowave to bend and be longer than in theuniform waveguide case. Such bending cannot be accounted for in ARTprocessing and so causes an error. But, by measuring the total pathphase shift, the bending effect can be accounted for in Full WaveInversion Code (FWIC), a tomography reconstruction algorithm. Thewaveguide attenuation effect reduces the magnitude of theelectromagnetic wave along the path.

[0036] It has been observed that the coal seam attenuation rate willincrease with signal frequency. The wavelength increases as frequencydecreases. By lowering the operating frequency, the measurement rangeincreases. But such range still depends on coal seam thickness, the typeof bounding sedimentary rock, and the electrical conductivity of thecoal.

[0037] Intrusions of sandstone sedimentary rock into the coal seamincrease the attenuation rate at that point. This is because more of thesignal travels vertically into the boundary rock and leaks from thewaveguide. If water has invaded the coal, then any wet clay in the coalwill cause the electrical conductivity to decrease and theattenuation-rate/phase-shift to increase. Such effects allow tomographyto be used to image such geologic anomalies.

[0038] The attenuation-rate/phase-shift rapidly increases withdecreasing seam height, so coal seam thinning can be easily detected.Higher attenuation-rate areas suggest that the coal seam boundary rockis changing, the seam is rapidly thinning, or/and water has invaded thecoal seam.

[0039] Faults and dykes in the coal seam will cause signal reflectionswithin the waveguide. Such reflections can appear as excess path loss,but can be differentiated as being caused by faults and dykes in thecoal by also taking phase shift measurements.

[0040] The preferred operating frequency band extends from a fewkilohertz to well over 300 kHz. The few kilohertz lower limit is due tothe practical problems in building low frequency antennas with theirlonger wavelengths, and the high frequency limit is due to the increasedcoal seam waveguide attenuation rate with frequency. Instrument designand tomographic image processing algorithms can greatly increase coalseam imaging resolution.

[0041] Tomographic ART images often have in the direction of the signalwave propagation. The ART algorithm assumes that the coal seam waveguidesignal travels on a straight ray path. Severe geologic anomalies cancause signal refraction, in which case the ray-path assumption fails.

[0042] Referring again to FIG. 2, the drillstring 212 preferablyincludes a radar system with both upward and downward looking antennasto non-invasively determine the vertical middle of coal layers 204 and206. Such further preferably includes a dielectric-constant measuringsensor and an inductive radio for communicating measured data to thesurface.

[0043] For example, the drillstring 212 includes at its distal endbehind drillhead 214 a measure-while-drilling (MWD) instrument with astepped-frequency radar operating in the 1.7 to 2.5-GHz range.Typically, fifty frequency steps are used to cover the frequency range.At each frequency step, in-phase (I) and quadrature (Q) radar returnsignal values are measured and transmitted to a user display at theground surface. The I and Q values for all step frequencies are decodedand processed in a Fast Fourier Transform (FFT) to derive the timedomain response from each of the roof and floor to the coal interface.The distances to each interface can then be determined from thesemeasurements by taking into account the relative dielectric constant.

[0044] Alternatively, since the object of steering the drillstring 212is to stay in the vertical middle of the coal seam 204 and 206, the rawmeasurements from the upward-looking and downward-looking radar antennascan be compared in their raw uncorrected state to see if they are aboutthe same. If so, it can be assumed that the measurements have occurredat the vertical middle, e.g., equidistant to ceiling and floor. Thecorrective information for each from the dielectric-constant sensor canin such case be dispensed with.

[0045] Accurate geologic data can be opportunistically acquired whiledrilling through such coal seams. But to do this, the radar-acquireddistance-data must be corrected for relative dielectric constant (∈r) todetermine the actual physical distance. In such case, theinstrumentation must include a local dielectric-constant measuringcapability.

[0046] The FFT computations needed for real-time MWD is verycomputer-intensive. With the present state-of-the-art inmicroprocessors, the measured data must be processed above ground wherelarger, more capable computers can be operated. Such computers alsodemand operating power that is too much for battery operation in thedrillstring 212. Therefore the measurements are communicated along thedrillstring, e.g., using inductively coupled radio communication.

[0047] Such upward-looking and downward-looking radar can also determinethe type of boundary rock in the roof and floor. Regions of high coalseam where the relative dielectric constant is high, e.g., greater than6.0, are unattractive because they produce greater amounts of water inthe methane drainage system. Any coal deposit set under a paleochanneland having a dielectric constant under 6.0 suggests that thepaleochannel is dry and will contain methane. Drilling near drypaleochannels increases methane production.

[0048] Conventional horizontal drilling navigation systems cannotdirectly determine seam thickness or changes in seam orientation, dipsand rolls, without trial-and-error exploratory drilling of the floor andthe roof in the same region of the panel. Thus, when the seam horizonchanges, the drill will impact the roof or floor. A drilling machineoperator with such conventional methods detects when the drill is on theroof or floor horizon by evidence of rock in the cuttings. The drillingtechnician redirects the drill motor to try to keep the drill within theseam. The borehole that results wanders between the roof and floor ofthe coal seam along its path. Punching through to roof or floorinterfaces can invite ground water to inundate the borehole.

[0049]FIG. 3 illustrates a radar-plow drillstring steering systemembodiment of the present invention, and is referred to herein by thegeneral reference numeral 300. The radar-plow drillstring steeringsystem 300 comprises a drill bit 302 on a distal end, a steering plow304, an electronics section 306 with retrieval docking, a repeatersection 308, and a drillstring section 310. The steering plow 304includes a set of four radially distributed pressure pads forup-down-left-right drilling control. Three of these pressure pads arevisible in FIG. 3, e.g., a top pressure pad 312, a bottom pressure pad314, and a side pressure pad 316. These are all controlled to assumevarious states of protrusion or retraction by a plow control 318. Ameasurements-while-drilling (MWD) instrument 320 processes radar signalsto-and-from horn radar antennas and dielectric-constant sensors embeddedin respective ones of the four radially distributed pressure pads. TheMWD instrument 320 provides estimates in real-time of the distances tothe boundary layers in the floor and ceiling of a coal seam. Theseestimates are used by the plow control to maintain a desired coursethrough the coal seam. A tether 322 is used to retrieve MWD instrument320 from within the drillstring back to the surface if the drill bit 302jams and cannot be retracted. A signal repeater 324 assists in longrange communication between the MWD instrument 320 and an operatordisplay and tomographic processor on the surface. A second tether 326 isused to retrieve the signal repeater 324.

[0050]FIG. 4 illustrates a drillstring radar system embodiment of thepresent invention, and is referred to herein by the general referencenumeral 400. The drillstring radar 400 comprises a downhole drillstring402 connected to a surface collar 404. A graphical user interface (GUI)406 is positioned at the surface and processes tomographic images of themeasurements-while-drilling on a user display 407. A microcomputer (CPU)408 does data logging and processes raw data received by a transceiver409 from the downhole equipment. A signal coupler 410 allows thetransceiver 409 to use the collar 404 and drillstring 402 as a commontransmission line. A radio repeater 411 repeats and amplifies “F1/F2”signals through its couplers 412 and 414. If the drillstring 402 isrelatively short, and signal attenuation is not severe, then repeater411 may not be needed. A measurements-while-drilling (MWD) instrument416 is connected to the radio signals on the drillstring 402 by acoupler 418. Coupler 418 is operated at about 100-kHz and typicalcomprises 14-turns of Litz-wire on a 12-inch long coil one inch wide andmounted edge onto the drill rod.

[0051] The MWD instrument 416 comprises a radio transceiver 422 that isconnected to an antenna 424. Such antenna 424 is operated at about125-kHz and typical comprises 14-turns of Litz-wire on a 24-inch longferrite rod one inch in diameter. It communicates with other radarimaging equipment collocated in another parallel borehole. A morecomplete description of how this antenna 424 is used and how associatedradar imaging equipment located in associated parallel boreholes can beused to an advantage in coal deposit imaging, is provided in otherUnited States patent applications of the present inventor, e.g., Ser.No. 10/259,912, filed Sep. 30, 2002, and titled, RADIO-IMAGING OFUNDERGROUND STRUCTURES. Such are incorporated herein by reference.

[0052] A processor (PIC) 426 interfaces the raw measurements from aradar electronics system 428 to the transceiver 422. A switching matrix430 allows the selection of an upward-looking radar horn antenna 432, aresonant microstrip patch antenna (RMPA) 434, and a downward-lookingradar horn antenna 436.

[0053] Such horn antennas are respectively embedded in the pressure padsassociated with the steering plow. Such are preferably operated in the2.0-2.5 GHz frequency band and have at least 20-dB of back-loberejection. A plow controller 438 provides control signals in real-timeto guide the direction of drilling according to measurements obtained bythe radar electronics 428.

[0054] Electrical power for the MWD instrument 416 can be provided by ahydro-generator 440 that taps into the hydraulic fluid flow through thedrillstring, or by a rechargeable battery 442, or both. Using both wouldallow data to continue to be collected and reported even though thehydraulic flow may have been stopped for some reason.

[0055] The MWD instrument 416 is preferably retractable should thedrillstring 402 become stuck in the drillhole. For example, the MWDinstrument is equipped with a self-docking mechanism to interconnectwith the antenna array subsection drill rod. A retractable capabilitypermits the MWD instrument and repeater to be hydraulically pumped-inand pulled out of the drillstring. Such a shuttle mechanism is describedby the present inventor in U.S. patent application, Ser. No. 10/161,378,filed Jun. 04, 2002, and titled SHUTTLE-IN RECEIVER FOR RADIO-IMAGINGUNDERGROUND GEOLOGIC STRUCTURES. Alternatively, a docking mechanismassociated with the MWD instrument 416 and its antenna array 420 allowsthe MWD instrument to be retrieved back inside the drillstring with atether should the drill head become hopelessly jammed or locked into theearth.

[0056] The antenna array 420 is preferably designed to accommodate theradar and dielectric microwave antennas, the radar transmitter antenna,and the data transmission antenna. For economy, the radar transmitterfunction can use the transmitter section of the data transceiver 422during periods that data is not being sent to the surface collar. TheMWD instrument is placed inside a beryllium-copper drill rod section,e.g., ten feet long. In one instance, the antenna array and MWDinstrument was located fifteen feet behind the downhole motor.

[0057] In general, embodiments of the present invention include methodsfor measuring a reflection port signal for a three-port directionalcoupler with its output port directly connected to the driving-point ofa conventional horn antenna. The reflection port is connected through afirst balanced mixer. A 0°/90° phase shifter rotates the signal phasebetween a forward coupling port of the three-port directional couplerand a second beat frequency balanced mixer. A first beat frequencybalanced mixer injection signal is phase coherent with the reflectionsignal, thereby canceling the phase shift of the PLL. The impedance ofthe sensor antenna in close proximity to natural media is made as closeas possible to the characteristic impedance of the three-portdirectional coupler, preferably fifty ohms. The beat frequency occurswith phase-coherent mixing in mixer M4 so as to cancel the phase driftof the crystal oscillator and create in-phase (I) and quadrature (Q)signals derived from the reflection port signal. Such can be used inremote sensing and imaging of non-metallic and metallic landmines.

[0058] One method of calibrating the sensor antenna uses sets ofpolynomial equations wherein the independent variable represents thevalue to be measured. Alternative embodiments include solving for thevalue of the independent variable (H) from a set of polynomial equationsthat is the difference between the calibration polynomial equation andthe measured values of I and Q. Alternative embodiments includedetermining the value of the independent variable (H_(o)) by independentmeans and using the difference between the measured and calibrationpolynomial value to detect and image the object. Other embodimentsdetermine the independent variable as being related to a materialthickness, dielectric constant, in-situ stress.

[0059] The measurements obtained from embodiments of the presentinvention can be tomographically processed to construct two andthree-dimensional images of the object, e.g., a silhouette image of theobject and its associated signal-to-noise ratio. The signal-to-noiseratio and the width of the reconstructed object are generally related tothe probability of the object's detection.

[0060] FIGS. 5A-5D illustrate a cross section of a drillstring steeringplow embodiment of the present invention, and such is referred herein bythe general reference numeral 500. The drillstring steering plow 500comprises a set of orthogonally distributed pressure pads 501-504. Anupward-looking radar horn antenna is embedded within pressure pad 501. Adownward-looking radar horn antenna is embedded within pressure pad 502.A resonant microstrip patch antenna (RMPA) operated as adielectric-constant sensor is embedded in either of side pressure pads503 and 504, or both. A corresponding set of servo motors 505-508 areused to adjust the relative protrudings of pressure pads 501-504. Thesepressure pads and servo motors can be coupled, for example, by jackscrews 509 and 510. Alternatively, hydraulic pumps and pistons can beused to forcefully position each of the pressure pads 501-504.

[0061]FIG. 5A shows the pressure pads 501-504 in their neutral positionfor straight ahead steering of the drillstring. FIG. 5B shows pressurepad 501 retracted and pressure pad 502 extended for upward steering ofthe drillstring. Such causes the drill bit to bite harder on the roof ofthe cut. FIG. 5C shows pressure pad 501 extended and pressure pad 502retracted for downward steering of the drillstring. Such causes thedrill bit to bite harder on the floor of the cut. FIG. 5D shows pressurepad 504 extended and pressure pad 503 retracted for steering thedrillstring to the left, assuming the view here is from behind. Suchcauses the drill bit to bite harder on the left side of the cut. In allsuch instances, the antennas and sensors maintain a very intimatecontact with the coal seam and squeeze out water in the gaps thatotherwise would interfere with imaging and sensing.

[0062] An RMPA antenna sensor is used to determine a relative dielectricconstant (∈_(C)) of any coal surrounding the drillstring rod radarsteering plow 500. The dielectric constant of coal is affected by howmuch moisture is in the coal. Upward and downward-looking radar horns518 and 520 each provide I and Q signals at each stepped frequency thatare interpreted by the microwave radar measurement circuit 514. Thedistance through the coal layer to a sedimentary interface can bedetermined from the radar data once the dielectric constant of thenatural media has been determined.

[0063]FIG. 6 illustrates a ground-penetrating radar system embodiment ofthe present invention, which is referred to herein by the generalreference numeral 600. Such could be used in MWD instrument 320 in FIG.3 or radar electronics 428 in FIG. 4. The system 600 is used tonon-invasively penetrate a coal seam 601 with microwave radio energy tofind a boundary rock interface 602. The system 600 includes a localoscillator 603 that produces a reference frequency (FX) with referencephase (θ₁) A first phase-locked loop (PLL) 604 synthesizes a radiofrequency F0, an integer harmonic of FX. The radio frequency F0 ispassed to a forward-coupling port of a power splitter 606. An outputport is connected to a wideband isolation amplifier 608. A three-portdirectional coupler 610 one-way couples the transmit signal out to adirectional radar antenna 612 which illuminates the coal seam 601 andproduces reflected waves from the boundary rock interface 602 with thecoal seam 601. A reflection port of the three-port directional coupler610 is used for measurements, e.g., where e_(R)=e_(O)Γ cos θ_(R), thereflected energy is a function of the output energy of antenna 612.

[0064] During operation, such directional radar antenna 612 is movedclose to a natural media or ground surface. A deliberate impedancemismatch is thus created that will result in higher levels of couplerreflection port output voltage or standing wave ratio (VSWR). Anyimpedance mismatches appearing at any of the ports on a directionalcoupler will reduce its directivity and isolation between ports.Wideband isolation amplifiers are used to stop reflected waves fromreaching the transmitter stages and causing impedance mismatches. Thebalanced mixers are in particular susceptible to performancedegradations under uncontrolled conditions.

[0065] Radar signals radiated from the directional radar antenna 612enter the natural media or ground and are reflected back attenuated andwith a change of phase. A wideband isolation amplifier 614 forwards thereflected-wave sample to a first balanced mixer (M1) 616.

[0066] A second PLL 618 synthesizes a coherent frequency F1, e.g., 10.70MHz. A sample of the transmitted signal is provided by the powersplitter 606 and a second wideband isolation amplifier 619 to aselectable 0°/90° phase shifter 620. A balanced mixer (M2) 622 providesan intermediate frequency (IF) F5 that is output by a third widebandisolation amplifier 624. The result is a suppressed-carrier signal withupper and lower sidebands offset from the carrier frequency by 10.70MHz. This in turn mixes with RF in balanced mixer (M1) 616 to produce afirst intermediate frequency (IF) F6. A bandpass filter 626 produces anoutput F6′. A third PLL 628 synthesizes another coherent frequency (F2),e.g., 10.720 MHz. This is combined with F6′ in a balanced mixer 630 toproduce a relatively low-frequency IF signal F7, e.g., 20.0 KHz.

[0067] Both the oscillator phase shift θ₁ and the frequency aremultiplied by N_(o) to create the output frequency N_(o)ω₁. The phaseshift N_(o)θ₁, is canceled on mixer M1. The coupler reflection portvoltage e_(R) dependence on load plane impedance is mathematicallyrepresented by Equation (3). The reflection coefficient (F) is a complexnumber that can be represented as a vector magnitude with phase angleθ_(R). Trigonometric identities are used and filter theory is applied tofind that,

F ₈ _(I) ¹ =kΓ cos θ_(R)

and

F ₈ _(O) ¹ =kΓ sin θ_(R)  (6)

[0068] The ratio of these direct current (DC) values and the inversetangent values solves for the reflection phase angle θ_(R). The constantk is determinable after instrument calibration. The magnitude of thereflection coefficient can be found from either of the above twoequations. The measured values of Γ and θ_(R) can be applied in Equation(3) to determine the load plane impedance.

[0069] A bandpass filter 632 produces a signal F7′. A fourth PLL 633synthesizes another radio frequency (F3). The radio frequency is appliedto the LO part of the fourth balanced mixer (M4) 634. Heterodyningproduces a signal output F8. An integrating filter 636 processes andoutputs a signal F8′. The phase drift (θ₁) in local oscillator 603 isautomatically cancelled and does not appear in the output. An analog todigital converter (ADC) 638 outputs a digital signal format, e.g., forfurther processing by a computer.

[0070] A control line 640 selects a 0° or 90° phase-shift through thephase shifter, and this will cause the system 600 to output in-phase orquadrature measurements, e.g., as represented by a DC output voltage ofthe ADC 638. This control line is typically connected to the samemicroprocessor that receives the digitized estimates from ADC 638. Sucharrangement provides time-multiplexed I and Q amplitude estimates thatare indexed to a calibration table, like FIG. 7, to find the depth tothe boundary rock interface 602 or the thickness of the coal seam 601covering the object. The microprocessor is preferably further providedwith control signals 641-644 so that the F0-F3 frequencies can bedigitally manipulated, e.g., for best transmission penetration andreceiver sensitivity on-the-fly as different kinds of media 601 andobjects 602 are inspected.

[0071] The estimates can be processed to provide landmine detection,guide drilling or excavation operations, control mining equipment, findlost buried objects in the soil, etc.

[0072] The three-port directional coupler 610 is preferably located nearthe RMPA antenna 612 and is connected to it by a short coaxial cable orstrip line. The RMPA antenna 612 is preferably placed close to a naturalmedia surface so the driving-point impedance of an antenna can beadjusted to match the characteristic impedance of the coupler. Thistechnique will maximize the sensitivity of the coupler reflection portsignal to small changes in antenna driving-point impedance. An antennadriving-point impedance adjustment is therefore preferred, e.g., with avariable slot built into the antenna structure capacitor.

[0073] The balanced mixer placed between the coupler reflected port andthe first balanced mixer is such that the signals that modulate thecoupler reflection output are phase coherent with the transmit signal.Sidebands are thus produced that represent the phase and amplitudeinformation in the reflection signals. The rest of the circuitrydemodulates the information from the carrier.

[0074] The output signal of the second balanced modulator (M2) is mixedwith a coherent sample of the transmit signal in the first balancedmixer. After filtering, such signal will faithfully replicate thereflection port signal. This simplifies the transceiver design andenables accurate signal measurements with low cost synchronous detectioncircuits.

[0075]FIG. 7 charts the relationships that typically develop between theRMPA antenna and the depth to the object and measured values for real(in-phase, I) and imaginary (quadrature-phase, Q). The two vectorcomponents of RMPA impedance, real and imaginary, vary differently as anearby media layer thickness changes. Plotting the imaginary on theY-axis and the real on the X-axis of a graph yields a calibration curve700 that spirals to a vanishing point with increasing layer thickness,e.g., from 0.25 to twelve inches.

[0076]FIG. 8 illustrates the theoretical impedance response 800, e.g.,as seen at the reflected-wave sample port of directional coupler 610.The applied signal (e_(o)) from isolation amplifier 608 is assumed to beconstant. Detection sensitivity is best on the steepest parts of thecurve. Sensitivity is maximum when the RMPA driving point impedancematches the characteristic impedance of the directional coupler.

[0077] A method embodiment of the present invention provides for thecalibration of such RMPA sensor antennas. Sets of polynomial equationsare constructed with independent variables (H) that allow any givenantenna driving point impedance value to be measured, and represented asa calibration function with independent variable (H). The differencebetween the measured and calibration polynomial value is used to detectand image the coal seam.

[0078] Method embodiments of the present invention can further definesuch independent variables to be thickness, dielectric constant, orin-situ stress of a geologic media or buried object. The informationgathered can be used to construct visual images of the coal seam and itscomposition.

[0079] Referring again to FIG. 6, the crystal oscillator phase shift θ₁is multiplied by the full multiplication factor N_(o) and the frequencyof the crystal oscillator is multiplied by N_(o) creating the outputfrequency N_(o)ω₁. After filtering, the phase shift N_(o)θ₁ is canceledon balanced mixer M1. The coupler reflection port voltage e_(R)dependence on driving point impedance is mathematically represented byEquation (3). The reflection coefficient (Γ) is a complex number thatcan be represented as a vector magnitude with phase angle θ_(R). Usingtrigonometric identities and applying filter theory, it can be shownthat,

F ₈ I=kΓ cos θ_(R)

and

F ₈ Q=kΓ sin θ_(R)  (12)

[0080] The ratio of these direct current (DC) values can be used tosolve for the inverse tangent values. And the reflection phase angleθ_(R) can be readily determined. Calibration of the instrument willdetermine the correct constant K. The magnitude of the reflectioncoefficient can be found from either of the above two equations. Thecircuits isolate the phase shifter from any blaring reflected signal.The measured values of Γ and θ_(R) can be applied in Equation (3) todetermine the driving point impedance.

[0081] The following equations help further describe the necessaryworking conditions of the system 600 in FIG. 6, and illustrate what eachfunctional element must do to process the signals involved. FR is thereflected-wave output from the reverse port of directional coupler 610.FX is the output of oscillator 603. F0 is the output of PLL 604. F1 isthe output of PLL 618. F2 is the output of PLL 628. F3 is the output ofPLL 633. F4I and F4Q are the selected in-phase and quadrature outputs ofphase shifter 620. F5 is the output of mixer 622. F6 is the output ofmixer 616. F6′ is the filtered output of bandpass filter 626. F7 is theoutput of mixer 630. F7′ is the filtered output of bandpass filter 632.F8 is the output of mixer 634. F8′ is the filtered output of bandpassfilter 636.

FX=A cos (ω₁ t+θ ₁)

F0=B cos (N₀ω₁ t+N₀θ₁)

F1=C cos (N₁ω₁ t+N₁θ₁)

F2=C cos (N₂ω₁ t+N₂θ₁)

F41=D cos (N₀ω₁ t+N₀θ₁)

F4Q=D sin (N₀ω₁ t+N₀θ₁)

FR=EΓ cos (N_(o)ω₁ t+N₀θ₁+θ_(R))

N₂−N₁=1

[0082] Placing the quadrature hybrid into the 0° position yields:${F5I} = {{{F1} \times {F4I}} = {{C\quad D\quad {\cos ( {{N_{1}\omega_{1}t} + {N_{1}\theta_{1}}} )}\cos ( {{N_{0}\omega_{1}t} + {N_{0}\theta_{1}}} )} = {\frac{C\quad D}{2}\lbrack {{\cos \lbrack {{( {N_{1} + N_{0}} )\omega_{1}t} + {( {N_{1} + N_{0}} )\theta_{1}}} \rbrack} + {\cos \lbrack {{( {N_{1} + N_{0}} )\omega_{1}t} + {( {N_{1} - N_{0}} )\theta_{1}}} \rbrack}} \rbrack}}}$${{F6I} = {{{F5} \times F\quad R} = {{\frac{CDE\Gamma}{2}\begin{bmatrix}\cos & {\lbrack {{( {N_{1} + N_{0}} )\omega_{1}t} + {( {N_{1} + N_{0}} )\theta_{1}}} \rbrack \times} \\\cos & {( {{N_{0}\omega_{1}t} + {N_{0}\theta_{1}} + \theta_{R}} ) +} \\\cos & {\lbrack {{( {N_{1} - N_{0}} )\omega_{1}t} + {( {N_{1} - N_{0}} )\theta_{1}}} \rbrack \times} \\\cos & ( {{N_{0}\omega_{1}t} + {N_{0}\theta_{1}t} + \theta_{R}} )\end{bmatrix}} = {\frac{CDE\Gamma}{2}\begin{bmatrix}\cos & {\lbrack {{( {N_{1} + {2N_{0}}} )\omega_{1}t} + {( {N_{1} + {2N_{0}}} )\theta_{1}} +  \theta_{R} )} \rbrack +} \\\cos & {( {{N_{1}\omega_{1}t} + {N_{1}\theta_{1}} - \theta_{R}} ) +} \\\cos & {( {{N_{1}\omega_{1}t} + {N_{1}\theta_{1}} - \theta_{R}} ) +} \\\cos & \lbrack {{( {N_{1} + {2N_{0}}} )\omega_{1}t} + {( {N_{1} + {2N_{0}}} )\theta_{1}} +  \theta_{R} )} \rbrack\end{bmatrix}}}}}$

[0083] After filtering: $\begin{matrix}{{F^{1}6I} = {\frac{CDE\Gamma}{4}\lbrack {{\cos ( {{N_{1}\omega_{1}t} + {N_{1}\theta_{1}} - \theta_{R}} )} + {\cos ( {{N_{1}\omega_{1}t} + {N_{1}\theta_{1}} + \theta_{R}} )}} \rbrack}} \\{= {\frac{CDE\Gamma}{2}{\cos ( \theta_{R} )}{\cos ( {{N_{1}\omega_{1}t} + {N_{1}\theta_{1}}} )}}} \\{{F7I} = {{{F2} \times F^{1}\sigma \quad I} = {\frac{C^{2}{DE\Gamma}}{4}{{\cos ( \theta_{R} )}\lbrack {{\cos ( {{N_{2}\omega_{1}t} + {N_{2}\theta_{1}}} )}{\cos ( {{N_{1}\omega_{1}t} + {N_{1}\theta_{1}}} )}} \rbrack}}}} \\{= {\frac{C^{2}{DE\Gamma}}{4}{{\cos ( \theta_{2} )}\begin{bmatrix}\cos & {\lbrack {{( {N_{2} + N_{1}} )\omega_{1}t} + {( {N_{2} + N_{1}} )\theta_{1}}} \rbrack +} \\\cos & \lbrack {{( {N_{2} - N_{1}} )\omega_{1}t} + {( {N_{2} - N_{1}} )\theta_{1}}} \rbrack\end{bmatrix}}}}\end{matrix}$

[0084] After filtering:${F^{1}7I} = {\frac{C^{2}{DE\Gamma}}{4}{\cos ( \theta_{R} )}{\cos \lbrack {{( {N_{2} - N_{1}} )\omega_{1}t} + {( {N_{2} - N_{1}} )\theta_{1}}} \rbrack}}$

[0085] where N₂−N₁=1, $\begin{matrix}{{F^{1}7I} = {\frac{C^{2}{DE\Gamma}}{4}{\cos ( \theta_{R} )}{\cos ( {{\omega_{1}t} + \theta_{1}} )}}} \\{{F8I} = {{F\quad X \times F^{1}7I} = {\frac{{AC}^{2}{DE\Gamma}}{4}{\cos ( \theta_{R} )}{\cos ( {{\omega_{1}t} + \theta_{1}} )}{\cos ( {{\omega_{1}t} + \theta_{1}} )}}}}\end{matrix}$

[0086] and applying filter:${F^{1}8I} = {\frac{{AC}^{2}{DE\Gamma}}{4}{\cos ( \theta_{R} )}}$

[0087] Switching the quadrature hybrid to the 90° position and followingsimilar techniques as shown above, reveals that:${F^{1}8Q} = {\frac{{AC}^{2}{DE\Gamma}}{4}{\sin ( \theta_{R} )}}$

[0088] Let I=F¹8Q and Q=F¹8Q and thus,

Γ∝{square root}{overscore (I²+Q²)}

and, $\theta_{R} = {\tan^{- 1}\frac{Q}{I}}$

[0089] Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that thedisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artafter having read the above disclosure. Accordingly, it is intended thatthe appended claims be interpreted as covering all alterations andmodifications as fall within the true spirit and scope of the invention.

What is claimed is:
 1. A plow steering system for directional drilling,comprising: a set of first through fourth pressure pads distributedaround the outside surface of a steering plow for placement behind adirectional drilling drill bit; an upward-looking radar antenna embeddedin said first pressure pad; a downward-looking radar antenna embedded insaid second pressure pad; a ground-penetrating radar transceiverconnected to each of the upward-looking and downward-looking radarantennas, and providing for return signal attenuation and phase shiftmeasurements, and for providing from said measurements real-timeestimates of the distances transmitted radar signals traveled beforebeing reflected back to a respective antenna; and a guidance controlproviding for steering of said directional drilling drill bit accordingto said real-time estimates, wherein said steering is provided byretracting and extending individual ones of said first through fourthpressure pads.
 2. The system of claim 1, further comprising: adielectric-constant sensor embedded in either of said third and fourthpressure pads, and providing corrective data to said real-time estimatesof the distances provided by the ground-penetrating radar transceiver.3. The system of claim 1, further comprising: a docking mechanism forremote disconnection of the ground-penetrating radar transceiver fromsaid directional-drilling drillstring; and a tether for causing thedocking mechanism to release the ground-penetrating radar transceiverand allowing it to be withdrawn from within the length of saiddrillstring.
 4. The system of claim 1, further comprising: a dockingmechanism for remote automatic connection of the ground-penetratingradar transceiver to said directional-drilling drillstring with the aidof hydraulic pumping to shuttle it to a distal end.
 5. The system ofclaim 1, further comprising: a user display for providing tomographicimages of a coal bed to an operator from data provided by theground-penetrating radar transceiver.
 6. The system of claim 5, furthercomprising: a signal repeater located along said drillstring between theuser display and the ground-penetrating radar transceiver and providingfor extended communication between them.
 7. The system of claim 1,further comprising: a navigation processor connected to sense the coursetaken by said directional-drilling drillstring and for providinginformation to the guidance control.
 8. The system of claim 1, furthercomprising: a hydroelectric generator for receiving a hydraulic flowthrough said drillstring and for powering the ground-penetrating radartransceiver.
 9. The system of claim 8, further comprising: a magneticclutch for coupling-in mechanical power to the hydroelectric generatorcollocated with the ground-penetrating radar transceiver inside anexplosion-proof enclosure.
 10. The system of claim 1, furthercomprising: an inductive coupler for supporting a radio communicationalong said drillstring acting as a transmission line.